Irreparable neurodegeneration characterizes many neurologic diseases and contributes to the decline in nervous system function with age. Thus, uncovering mechanisms for neuronal replacement is a central challenge of therapeutics for many clinical presentations. This challenge is formidable in the CNS because of the variety and complexity of neurons, glia and their connections. Therefore, we are investigating a simpler model: the neuron-like sensory hair cells (HCs) of the auditory system, which are supported by glia-like cells and relay sound stimuli through auditory neurons to the brain. Genetic and environmental damage to HCs cause sensorineural hearing loss (SNHL), a common condition with significant healthcare costs. Regardless of initiating cause, in most cases of SNHL, the HCs die, whereas supporting cells (SCs) and auditory neurons can persist. However, as mammalian cochleae have no regenerative capacity, loss of HCs leads to permanent SNHL that is ameliorated, but not cured, by current therapies. Inhibition of Notch signaling has emerged as a means of inducing new HCs from residual SCs in deafened animals, but in mice it is inefficient after the embryonic stages when HCs normally arise, suggesting that regulation of additional developmental signals may be critical to HC regeneration and rewiring. Among the critical signals required for genesis of both HCs and SCs are members of the Fibroblast Growth Factor (FGF) family, which differ in their ability to activate different FGF receptors. FGF20/FGFR1 signaling promotes development of outer hair cells (OHCs), which amplify sound stimuli transduced by inner hair cells (IHCs); and FGF8/FGFR3 signaling promotes differentiation of pillar cells (PCs), SCs that separate IHCs from OHCs. Curiously, although genetic loss and gain of FGFR3 signaling have opposing effects on PC differentiation that cause SNHL, both changes induce ectopic OHCs supported by ectopic Deiters' cells (DCs). Fgfr3-/- mice show ectopic OHCs and DCs by late gestation. Our preliminary studies of a mouse FGFR3 gain-of-function mutation with altered ligand-binding specificity show that ectopic OHCs and DCs arise postnatally. Furthermore, their genesis depends on FGF10, the same unexpected ligand that alters RAS/MAPK target genes and induces the DC-to-PC fate transformation and plasticity we described previously. In this proposal we test the hypothesis that FGF/RAS/MAPK signaling is sufficient to induce OHC differentiation from DC progenitors both embryonically and postnatally, including in an in vivo model of OHC loss. We will 1) determine whether an ectopic gain-of-function mechanism drives ectopic OHCs and DCs in Fgfr3-/- mice; 2) determine the lineage of ectopic OHCs in FGFR3 loss- and gain-of-function mutants; and 3) determine whether forced activation of RAS/MAPK signaling in DCs of normal and OHC-depleted cochleae will induce OHCs. Our results will drive future studies addressing the long-term goal of harnessing the FGF pathway together with other signals to induce robust mammalian auditory HC regeneration and hearing restoration in damaged cochleae, and will inform efforts to manipulate CNS glia to differentiate as neurons.